Seasons
Climatic differences throughout the world are caused in the first instance by the differing amounts of sunlight received at different latitudes of the Earth and at different times of the year. More sunlight is received nearer the equator than near the poles where the angle of the midday Sun is greater.
During the course of the Earth's orbit around the Sun (a year), the angle of maximum incidence of the Sun at the Earth's surface changes. This is due to the tilt of the Earth's orbit, 23.5° from the perpendicular. Warmest temperatures at a particular location on the Earth occur when that location is tilted towards the Sun, during the summer. In addition, the number of daylight hours is greater, increasing the time available for solar heating. Winter occurs when that part of the Earth's surface is tilted away from the Sun, the angle of the midday Sun is lower and there are fewer daylight hours. Consequently, summer and winter occur at opposite times of the year in the Northern and Southern Hemispheres. During the spring and autumn equinoxes in March and September, every place on Earth receives 12 hours of daylight and 12 hours of darkness.
Changes in the angle of the midday Sun through the seasons have the greatest effect in the mid latitudes where the contrasts between winter and summer can be greatest. Although maritime climates in the mid latitudes are moderated by the influence of the oceans, within the continental interiors, winter temperatures can fall to -40°, whilst summer temperatures often exceed 30°C.
In the polar regions, the angle of the midday Sun is never very high above the horizon, and summer temperatures remain close to or even below freezing. At the poles, sunlight may shine continuously for six months, but never rises more than 23.5° above the horizon. During the other six months of the year, the poles are in complete darkness, and temperatures of -70°C and lower have been recorded.
Within the tropics, the angle of incidence of sunlight remains relatively high throughout the year and seasonal patterns of temperature are not evident. Instead, the year is divided up into wet and dry seasons. Wet seasons occur during the months of greatest solar heating when the midday Sun is overhead, generating significant vertical uplift or convection of air that is accompanied by the almost daily formation of large thunderstorms. This zone of convection is called the Inter-Tropical Convergence Zone (ITCZ), which moves with the seasons north and south of the equator between the Tropics of Cancer (Northern Hemisphere) and Capricorn (Southern Hemisphere). Close to the equator, the ITCZ influences the weather twice a year during the equinoxes in March and September. Near the Tropic of Cancer, the ITCZ approaches only during June and July, and climates at these latitudes generally experience only one wet season and a prolonged dry season throughout the remainder of the year. Near the Tropic of Capricorn, the short wet season occurs during December and January. In some parts of the world, for example India, the special pattern of atmospheric pressure and wind which accompanies the wet season, is known as the monsoon.
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